This circuit uses the ATtiny 8-pin microcontroller which has 5 I/O pins to create a 7-segment display. Since a 7-segment display only requires control of 7 individual LEDs, we use 4 of the ATtiny I/O pins as charlieplexed outputs (n*(n-1)). Leaving the the fifth I/O pin to be used as digital or analog input or another output.